Rts1 is a high-molecular weight drug resistance factor which has a molecular weight of 126.times.10.sup.6 daltons and confers resistance to kanamycin. In addition to conferring kanamycin resistance (Km.sup.r), the plasmid exhibits unusual temperature-sensitive phenotypes, which affect plasmid maintenance and replication, as well as host cell growth. These phenotypes include growth inhibition of gram negative bacteria harboring the plasmid at 42.degree. C., but not at 32.degree. C. (the so-called tsg, temperature sensitive growth). In addition, E. coli cells harboring the plasmid allow T4 phage growth at 42.degree. C., yet phage growth is restricted at 32.degree. C. Furthermore, the replication cycle of this plasmid at 42.degree. C. appears to bypass the typical covalently closed circular (CCC) form and proceeds (the so-called Tsc phenotype, temperature sensitive circular DNA formation). At 42.degree. C., but not at 32.degree. C., conjugative transfer of Rts1 is inhibited. Finally, prolonged incubation of bacteria harboring this plasmid at 42.degree. C. results in plasmid loss from host cells (the so-called Tdi, temperature dependent instability).
The nucleotide sequence of Rts1 has been elucidated, and partial nucleotide sequences have been reported, including those regions involved in DNA replication, incompatibility, kanamycin resistance and temperature-dependent instability of the plasmid. A particular region of the plasmid, known as the "repA" region has been identified and the sequence elucidated. The repA protein has been shown to be important in determining the copy number of the Rts1 plasmid maintained in a host cell. Mutated Rts1 plasmids having mutations in the repA region have been described, but induce only a moderate change in the copy number of the plasmid (i.e. at most 5 to 10-tenfold increases).
A second plasmid, known as P1, has been described in the art and contains a similar repA region.
No mutants of these two plasmids, however, are known in the art which induce significant increased copy numbers or which are easily controllable by changes in growth media.